The present invention relates to load control circuits, for example, lamp dimming circuits, and in particular, to an improved load control circuit for reducing acoustic noise, particularly in connection with dimming control of transformer-supplied lighting loads. The invention can also be used to control the speed of electrical motors for applications such as fans, motorized window treatments, and electrical tools, such as drills, grinders, and sanders.
Low-voltage lighting, for example, halogen lighting, has come into increased use in recent years. These lamps operate on low voltages, for example 12 volts or 24 volts, and accordingly, a transformer is employed to reduce the normal line voltage to the low voltage necessary to operate the lamps.
There has been an increase in complaints about acoustic noise by customers operating such lamps. The acoustic noise is believed to result from a number of factors including: the use of low-profile transformers in the same space as the lights, the increase in the use of toroidal transformers (versus “coil and core” transformers, such as transformers having EI cores, which have laminated cores made from E-shaped and I-shaped pieces), and the increase in use of open wire or rail low-voltage lighting in residential applications. Primarily, the increase appears to be due to the use of large VA (volt-ampere) toroidal transformers (typically, in the range of 150–600 VA).
Acoustic noise has always been an issue with magnetic low-voltage (MLV) loads. A lamp debuzzing coil or choke placed in series with the transformer primary winding reduces or eliminates the noise by increasing the rise time of the current. However, this solution has proved inadequate in view of the above factors now often present in the implementation of low-voltage lighting. It appears that one of the reasons for the acoustic noise is that the transformer saturates more easily due to direct current (DC) components in the input waveform. This is particularly a problem when the transformer has little or no air gap, such as is true of toroidal transformers.
There is accordingly a need for an improved load control circuit, and in particular, a dimmer circuit for low-voltage lighting and in applications where there are MLV loads, in order to reduce the generation of acoustic noise.
FIG. 1 shows a typical prior art two-wire phase-cut (sometimes referred to as “phase-control”) dimming circuit 100. Dimming circuit 100 is known as a two-wire dimmer because the only connections necessary are the HOT terminal 102, which is connected to a first terminal of a source of line frequency alternating current (AC) voltage 104, and the DIMMED HOT terminal 106, which is connected to a first terminal of a load 108. A second terminal of the load 108 is connected to a second terminal of the AC voltage source 104 to complete the electrical path. The dimmed hot output voltage comprises a phase-cut AC voltage waveform, as well known to those of skill in the art, wherein current is only provided to the lamp load after a certain phase angle of each half cycle of the AC waveform.
In order to accomplish this, a triac 110 is employed to control the amount of voltage delivered to the load 108. A timing circuit 120 comprises a double-phase-shift resistor-capacitor (RC) circuit having a resistor R122, a potentiometer R124, and capacitors C126, C128. The timing circuit 120 sets a threshold voltage, which is the voltage across capacitor C128, for turning on the triac 110 after a selected phase angle in each half cycle. The charging time of the capacitor C128 is varied in response to a change in the resistance of potentiometer R124 to change the selected phase angle at which the triac conducts. A diac 130 is in series with the control input, or gate, of the triac 110 and is employed as a triggering device. The diac 130 has a breakover voltage (for example 30V), and will pass current to the triac gate only when the threshold voltage exceeds the breakover voltage of the diac plus the gate voltage of the triac. The prior art circuit also employs an input noise/EMI filter stage comprising an inductor L142, a resistor R144, and a capacitor C146.
Another prior art circuit 200 is shown in FIG. 2A. This circuit employs a voltage compensation circuit 250, including a diac 252 and a resistor R254, to adjust the voltage to the potentiometer R224 to compensate for line voltage amplitude variations. As is well known, diacs have a negative impedance transfer function so that, as the current through the diac decreases, the voltage across the diac increases. As the voltage across the dimmer decreases, the current through the diac 252 also decreases. As a result, the voltage across the diac 252 increases, causing the current flowing through R224 to C228 to increase, thereby causing capacitor C228 to charge to the threshold voltage sooner. This results in increased conduction time for triac 210 to compensate for the decreased voltage across the dimmer, thereby maintaining the set light level.
In addition, the prior art circuit shown in FIG. 2A includes a DC voltage correction circuit 260, including a capacitor C264 and a resistor R262, to maintain a net average output voltage of zero volts DC. The operation of the DC voltage correction circuit is described in U.S. Pat. No. 4,876,498, the entirety of which is incorporated by reference herein, and hence, will not be further described here.
The prior art devices of FIGS. 1 and 2A have been known to cause excessive acoustic noise to be generated in a load, such as an MLV lamp load, comprising a transformer-supplied low-voltage lamp, when such a load is coupled to the output of the dimmer.
FIG. 2B shows the waveform of the voltage across a 600 VA toroidal transformer provided by the prior art circuit of FIG. 2A. The waveform shows asymmetry in the two half cycles. Asymmetry, as used herein, means that the conduction time of the triac in the positive half cycle, t2(POS), is not equal to the conduction time of the triac in the negative half cycle, t2(NEG). As a result, the area under the curve of the voltage across the load (measured in volt-seconds) during the positive half cycle is not equal to the area under the curve of the voltage across the load (measured in volt-seconds) during the negative half cycle. This asymmetry results in the output voltage having a net DC component. It is believed that this asymmetry causes the transformer to saturate, thereby increasing acoustic noise. The voltage overshoot shown in FIG. 2B, in the portion labeled A, indicates that the transformer is saturating as a result of the asymmetry in the output voltage waveform. In this case, a lamp debuzzing coil or choke will be unable to eliminate acoustic noise from the transformer, resulting from asymmetry in the output voltage, because the coil or choke does not eliminate the net DC component.
FIG. 3A shows the schematic of another prior art circuit comprising a three-wire dimmer 300 having a terminal connection NEUTRAL for direct connection to the neutral line of an AC voltage source. This circuit has a similar structure to the prior art circuit of FIG. 2A, and includes a triac 310, a timing circuit 320, a trigger circuit 330, a voltage compensation circuit 350, and a DC correction circuit 360. Timing circuit 320 includes a potentiometer R324, for setting the desired conduction time for the triac 310 and hence, the desired output voltage for the dimmer 300, and a capacitor C328 that charges to a threshold voltage. Trigger circuit 330 includes a current amplifier consisting of diodes D331, D332, and transistors Q333, Q334, a full-wave bridge rectifier consisting of bridge BR335, resistors R336, R337, a threshold device consisting of silicon bilateral switch 338, an optocoupler 339, and resistors R340, R341. The optocoupler 339 provides electrical isolation between NEUTRAL and the triac 310. The bridge BR335 allows current to flow through the photodiode 339A of the optocoupler 339 in the same direction during both half cycles of the AC line voltage. The silicon bilateral switch 338 allows current to flow through the photodiode 339A only when the voltage across capacitor C328 reaches a threshold value.
It has been discovered that the circuit of FIG. 3A causes less acoustic noise than the circuits of FIGS. 1 and 2A. FIG. 3B shows the output waveform of the circuit of FIG. 3A, showing how it is more symmetrical, with a smaller DC component. The three-wire dimmer of FIG. 3A has a more symmetrical output waveform because the presence of the neutral connection allows the timing circuit 320 to be decoupled from the load. The timing circuit 320 of the three-wire dimmer charges from the HOT terminal through the timing circuit 320 to the NEUTRAL terminal. In contrast, the timing circuit 220 of the two-wire dimmer of FIG. 2A charges from the HOT terminal through the timing circuit 220 to the DIMMED HOT terminal, then through the load to the neutral connection of the AC voltage source.
It has been realized that if the conduction times of the bidirectional switch of a two-wire load control circuit are the same in the positive and negative half cycles, then the output voltage waveform exhibits greater symmetry, and hence, a reduced DC component. It is believed that asymmetries in the voltage and current characteristics of both the diac and the triac in their respective modes of operation contribute to the asymmetry and DC component of the output waveform. In particular, three sources of asymmetry have been identified: (1) the breakover voltage of the diac in a first direction is not equal to the breakover voltage of the diac in a second (opposite) direction; (2) the voltage-current characteristic of the diac when conducting in the first direction is not equal to the voltage-current characteristic of the diac when conducting in the second direction; and (3) the current into the gate of the triac at turn-on in a first direction is not equal to the current out of the gate of the triac at turn-on in a second (opposite) direction.
Referring to FIG. 3C, there may be seen the voltage-current (V-I) characteristic for a diac. It has been discovered that the V-I characteristics for diacs operating in the first quadrant are seldom (if ever) symmetric with the V-I characteristics for the same diacs operating in the third quadrant. For example, VBO+, which is the breakover voltage of the diac in the first (or forward) direction of conduction, may not be equal in magnitude to VBO−, which is the breakover voltage of the diac in the second (or reverse) direction of conduction. Unequal magnitudes of breakover voltage particularly affect the charging time of the capacitor C228 shown in the two-wire dimmer of FIG. 2A.
The shapes of the V-I characteristics in the first (I) and third (III) quadrants of operation, and in particular, the magnitudes of the breakback voltages, VBB+ and VBB−, affect the level to which the capacitor C228 ultimately discharges. If these V-l characteristics are not perfectly symmetrical, then the capacitor C228 may not discharge to the same point at the end of each half cycle of the line cycle. This can result in the initial conditions of capacitor C228 not being the same at the beginning of each half cycle. Accordingly, capacitor C228 will not consistently charge to the desired threshold voltage in the same amount of time from half cycle to half cycle.
Referring to FIG. 3D, there may be seen therein the waveform, −VC228, for the voltage across the capacitor C228, and a waveform, IGATE, of the gate current of the triac of the two-wire dimmer of FIG. 2A. In FIG. 3D, the vertical voltage scale is 20 V/div, the vertical current scale is 0.5 A/div, and the horizontal time scale is 2 ms/div. In the Figure, the polarity of the capacitor voltage VC228 has been reversed for ease of viewing. It will be appreciated that, at the moment the triac begins conducting, a spike of current, SI (of about 0.65 A), flows in to the triac gate lead when the triac begins conducting in the first (or positive) direction (corresponding to conduction in quadrant I), and a spike of current, SIII (of about 1.1 A), flows out of the triac gate lead when the triac begins conducting in the second (or negative) direction (corresponding to conduction in quadrant III). Thus, it may be seen that the current flowing out of the triac gate during the negative half cycle is nearly twice as large as the current flowing into the triac gate during the positive half cycle. Inequality in the magnitudes of the current spikes in the two directions results in the capacitor C228 discharging to different levels at the ends of each half cycle, which in turn results in the initial conditions of C228 being different at the beginning of the following half cycle. Differences in the initial conditions of capacitor C228 cause the conduction time of the triac to be different from one half cycle to the next half cycle.
Accordingly, there is a need for a two-wire load control circuit that supplies a symmetric voltage waveform, with substantially no DC component, to an MLV load, such as a transformer-supplied lamp load. In particular, there is a need for-a two-wire dimmer having a diac and a triac in which asymmetries in the diac and the triac have been substantially reduced or eliminated.